Gary Thompson

The thriller "Sanctum" is being sold under the banner of James Cameron, though all he really did was lend out his proprietary 3-D "Avatar" cameras.

That's the good news - the 3-D is strong and well-suited to the movie's story of a diving expedition gone horribly wrong (this is not a horror movie, as the ad campaign implies).

The not-so-good news is that "Sanctum" has channeled Cameron's weakness for dialogue that's corny, and in some cases worse than corny. As tragic events unfold, the most the survivors can muster, by way of benediction, is "f---" or "s---."

You might be tempted to do the same, as you realize that you've invested your 3-D-price premium in a movie that so desperately needs a rewrite.

"Sanctum" is loosely based on a diving expedition into famous caverns in New Guinea. Divers explore a massive series of connected underwater caves, hoping to discover new chambers and to experience the thrill of being the first humans to gaze upon the massive, cathedral-like spaces. (The movie, shot on location, is frequently gorgeous.)

The work is exceedingly dangerous, as we see immediately. Even experienced divers have mishaps, leading to split-second decisions that require merciless, life-and-death calculations.

Making most of these decisions is the dive-team leader, a leather-tough Aussie named Frank (Richard Roxburgh), who's like Paul Hogan without a sense of humor (throw another dead diver on the barbie).

He directs dives with unquestioned authority, at least until the arrival of the expedition's billionaire sponsor (Ioan Gruffudd), a feckless risk-taker who drags his trophy gal pal (Alice Parkinson) along for a first-person inspection.

It's a disaster movie, so of course disaster strikes - flash floods that swamp the upper chambers and strand the explorers, who have no choice but to go deeper to survive, looking for air pockets, or a new way out.

The stunt work is first-rate. All scenes of physical action are exciting and convincingly perilous, as the men and women dodge columns of deadly water, climb over slick rocks, and manage diminishing supplies of air, food, water, light.

The movie's characters, alas, are thinly drawn and tend to lose dimension and credibility as the story unfolds. The money man - surprise! - turns out to be a callow dilettante who's (pardon the pun) way out of his depth, etc.

Then there is the matter of the movie's crazed moral compass. "Sanctum" does not seem to realize that the expedition leader is taking his survivalist ethos to demented extremes (the movie is awash in suicides and mercy killings), and one bizarre moment of father-son bonding occurs as dad passes along the family tradition of euthanasia. Yikes.

The movie does not trust its human relationships, and so conjures a third-act villain, who must be killed twice, a sure sign that the movie has surrendered to cheap thrills.